A latent electrostatic image can be developed with toner particles dispersed in an insulating non-polar liquid. Such dispersed materials are known as liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by toner particles dispersed in a non-polar liquid. The image may then be transferred to a receiver sheet.
Useful liquid toners comprise thermoplastic resin toner particles and a dispersant non-polar liquid. Generally, a suitable colorant, such as a dye or pigment, is present in the toner particles. The colored toner particles are dispersed in a non-polar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant (i.e. below 3.0) and a high vapor pressure. Generally, the toner particles are less than 30 .mu.m average by area size as measured using the Malvern 3600E particle sizer.
Liquid developers are typically produced as 10-20% by weight solids concentrate. However, liquid developers which are particularly useful are those with high solids content, because the carrier liquid which is contained within the machine tends to build up as developer containing a high percentage of carrier liquid is added. The more concentrated the developer used initially, the less carrier liquid builds up in the machine. Liquid which builds up in the machine must be disposed of as chemical waste, which is undesirable and inconvenient for the user.
U.S. Pat. No. 5,019,477 to Felder discloses a liquid electrostatic developer comprising a non-polar liquid, thermoplastic resin particles, and a charge director. The thermoplastic resin particles comprise a mixture of: (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1-20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyltoluene and styrene and (iv) selected from the group consisting of butadiene and acrylate. As the copolymer of polyethylene and methacrylic acid or methacrylic acid alkyl esters, Nucrele.RTM. may be used.
U.S. Pat. Nos. 3,852,208 and 3,933,664, both to Nagashima et al., disclose colored, light-transparent photoconductive material which is obtained by a condensation reaction of organic photoconductive substances with reactive colored components. The chemical combination of an organic photoconductive substance having at least one amino or hydroxyl group with a color development component having at least one active halogen atom produces the color developing organic photoconductive materials. Alternatively, the color developing materials can be obtained from the combination of an organic photoconductive substance having at least one active halogen atom with a color developing component having at least one amino or hydroxyl group. The color developing organic photoconductive material may be pulverized in a ball-mill, a roll-mill or an atomizer to produce a toner for use as a dry or wet developing agent, or may be used in combination with other colored substances or vehicle resins.
U.S. Pat. No. 4,524,119 to Luly et al. discloses dry electrophotographic development carriers for use with toner particles wherein the carrier core particles are coated with fluorinated carbon or a fluorinated carbon-containing resin. By varying the fluorine content of the fluorinated carbon, systematic uniform variation of the resistivity properties of the carrier is permitted. Suitable binders for use with the carrier core particles may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 to Tsubuko et al. discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner, so that the toner is provided with both liquid-repelling and solvent-soluble properties.
U.S. Pat. No. 5,030,535 to Drappel et al. discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C.-120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 4,762,764 to Ng et al. and U.S. Pat. No. 4,476,210 to Croucher et al. disclose a liquid developer comprising an amphipathic stabilizer polymer irreversibly anchored to a thermoplastic resin core of marking particles. The stabilizer has a soluble polymer backbone with an insoluble anchoring chain grafted onto the polymer backbone. The stabilizer may comprise an AB or ABA type block copolymer. The block copolymers may include siloxanes. The procedure for preparing the liquid developer comprises the steps of: (1) preparation of the amphipathic stabilizer; (2) non-aqueous dispersion polymerization of the core monomer in the presence of the amphipathic stabilizer to provide stabilized particles; (3) dyeing of the non-aqueous dispersion particles; and (4) negatively charging the particles.
A need continues to exist for an extended range of liquid developers with the desirable property of preventing fluid buildup in the machine and easy redispersion of toner solids at the time of use.